Napa artist Stephen Whisler says he is the person who put up signs last week on Highway 101 and Highway 37 in Marin reading "Speed Enforced By Drones," complete with a missile firing from the craft.

Whisler said the prank was a political statement on the controversial drones -- used for spying and warfare -- and a general comment on government spy activities.

"I wanted to start a conversation," Whisler, 59, said. "On the one hand, the signs are funny, but they also address a very serious topic, surveillance, with the whole NSA thing and these drones being used all over the world."

The signs in Marin, discovered July 15, were found on eastbound Highway 37 near Novato Creek and along north and southbound Highway 101 at the Marin City/Sausalito exit. A fourth was placed along Highway 37 near Highway 121.

"I wanted to get as much exposure as I could, that's why I did Highway 101 at Sausalito," Whisler said. "I thought about Berkeley, but that was too crowded. I thought I might get caught."

Whisler said he and an unnamed accomplice set out in broad daylight and put up the signs without any problem.

"We dressed in reflective vests and hard hats and put out cones," he said. "I also have a white pickup truck, something that a road crew would use."

The California Highway Patrol is investigating the case. It is a violation of the California Vehicle Code to put up unauthorized signs on highways and Whisler could face a penalty.

"You can't help but get a chuckle, it's pretty entertaining," said CHP Officer Andrew Barclay. "I can appreciate the humor behind it."